Enable AVX-512 for string/span Equals/StartsWith

[EgorBo]

Contributes to #77034

A small clean up in impExpandHalfConstEqualsSIMD to make it SIMD size agnostic. At some point I plan to unify this with LowerCallMemcmp but currently they're different: this one has OrdinalIngoreCase support and produces better codegen when data is half constant (not just length). LowerCallMemcmp will aslo be removed because we need a new phase where we can rely on SSA/VN to keep all existing optimizations.

Benchmark:

string _testData = // field
    "https://github.com/dotnet/runtime/blob/main/CONTRIBUTING.md?r=0";

[Benchmark]
public bool Test_IgnoreCase() => _testData.StartsWith(
    "https://github.com/dotnet/runtime/blob/main/CONTRIBUTING.md", 
    StringComparison.OrdinalIgnoreCase);

[Benchmark]
public bool Test() => _testData.StartsWith(
    "https://github.com/dotnet/runtime/blob/main/CONTRIBUTING.md",
    StringComparison.Ordinal);

Method	Toolchain	Mean	Ratio
Test_IgnoreCase	\runtime-PR\corerun.exe	1.1675 ns	1.00
Test_IgnoreCase	\runtime-base\corerun.exe	8.5046 ns	7.28
Test	\runtime-PR\corerun.exe	0.7078 ns	1.00
Test	\runtime-base\corerun.exe	2.9632 ns	4.19

(Ryzen 7950x, win11-x64)

codegen diff for Test_IgnoreCase():

Was:

; Method Prog:Test_IgnoreCase():bool:this
       mov      rcx, gword ptr [rcx+08H]
       mov      rdx, 0x276002095A8      ; 'https://github.com/dotnet/runtime/blob/main/CONTRIBUTING.md'
       mov      r8d, 5
       cmp      dword ptr [rcx], ecx
       tail.jmp [System.String:StartsWith(System.String,int):bool:this] ;; not unrolled, size is too big for AVX1
; Total bytes of code: 28

New codgen:

; Method Prog:Test_IgnoreCase():bool:this
       vzeroupper 
       mov      rax, gword ptr [rcx+08H]
       cmp      dword ptr [rax+08H], 59
       jl       SHORT G_M52390_IG04
       vmovups  zmm0, zmmword ptr [rax+0CH]
       vporq    zmm0, zmm0, zmmword ptr [reloc @RWD00]
       vpxorq   zmm0, zmm0, zmmword ptr [reloc @RWD64]
       vmovups  zmm1, zmmword ptr [rax+42H]
       vporq    zmm1, zmm1, zmmword ptr [reloc @RWD128]
       vpxorq   zmm1, zmm1, zmmword ptr [reloc @RWD192]
       vporq    zmm0, zmm0, zmm1
       vxorps   zmm1, zmm1, zmm1
       vpcmpuq  k1, zmm0, zmm1, 0
       kortestb k1, k1
       setb     al
       movzx    rax, al
       jmp      SHORT G_M52390_IG05
G_M52390_IG04:  
       xor      eax, eax
G_M52390_IG05:  
       vzeroupper 
       ret      
RWD00  	dq	0020002000200020h, 0000000000000020h, 0020002000200020h, 0020000000200020h, 0020000000200020h, 0020002000200020h, 0020002000000020h, 0020002000200020h
RWD64  	dq	0070007400740068h, 002F002F003A0073h, 0068007400690067h, 0063002E00620075h, 0064002F006D006Fh, 0065006E0074006Fh, 00750072002F0074h, 006D00690074006Eh
RWD128 	dq	0020002000200020h, 0020000000200020h, 0000002000200020h, 0020002000200020h, 0020002000200000h, 0020002000200020h, 0020002000200020h, 0020002000000020h
RWD192 	dq	00690074006E0075h, 0062002F0065006Dh, 002F0062006F006Ch, 006E00690061006Dh, 006E006F0063002Fh, 0062006900720074h, 006E006900740075h, 0064006D002E0067h
; Total bytes of code: 110

AVX-512 is used for [32..64] length range (in utf16)

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Issue Details

---

Contributes to #77034

A small clean up in impExpandHalfConstEqualsSIMD to make it SIMD size agnostic. At some point I plan to unify this with LowerCallMemcmp but currently they're different: this one has OrdinalIngoreCase support and produces better codegen when data is half constant (not just length). LowerCallMemcmp will aslo be removed because we need a new phase where we can rely on SSA/VN to keep all existing optimizations.

Benchmark:

private string TestData => 
    "https://github.com/dotnet/runtime/blob/main/CONTRIBUTING.md?r=0";

[Benchmark]
public bool Test() => TestData.StartsWith(
    "https://github.com/dotnet/runtime/blob/main/CONTRIBUTING.md", 
    StringComparison.OrdinalIgnoreCase);

codegen diff for Test(): https://www.diffchecker.com/5n4oF6gn/

Method	Toolchain	Mean	Ratio
Test	\runtime-base\corerun.exe	0.9315 ns	1.00
Test	\runtime-PR\corerun.exe	8.7793 ns	9.50

10x faster (Ryzen 7950x, win11-x64)

Author:	EgorBo
Assignees:	EgorBo
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

/benchmark json aspnet-citrine-win runtime

[pr-benchmarks]

Benchmark started for json on aspnet-citrine-win with runtime. Logs: link

[EgorBo]

@tannergooding @BruceForstall @dotnet/avx512-contrib PTAL, no impact on TE, SPMI diffs are empty due to missing contexts - lots of missing contexts mean it found quite a few places for avx512 path, jit-utils found +16kb diff on BCL.

[tannergooding]

This will also hit #84912, but @EgorBo said he plans on fixing both issues at once in a single PR, that will help avoid additional CI churn and mixing dissimilar changes in.